1. Field of the Invention
The present invention relates to an improvement in a process for preparing a well-ordered crystalline thin film of copper-containing superconducting compound oxide such as YBCO on a silicon wafer substrate, more particularly, to a process for pre-treating a surface of the silicon wafer substrate.
2. Description of the related art
The superconducting phenomenon has been explained as a unique phenomenon which can be observed only at extremely low temperatures which can be realized with liquid helium. However, a new type superconducting compound oxide of (La, Sr).sub.2 CuO.sub.4 which exhibits superconductivity at 30.degree. K. was reported in 1986 by Bednorz and Muller (Z. Phys. B64, (1986) 189). Then, another superconducting material of YBa.sub.2 Cu.sub.3 O.sub.y having a higher critical temperature of about 90.degree. K. was reported in 1987 by C.W. Chu et al. (Physical Review Letters, Vol. 58, No. 9, p. 908) and then Maeda et al. reported a so-called bismuth type compound oxide superconductor having a critical temperature of about 100.degree. K. in 1988 (Japanese Journal of Applied Physics. Vol. 27, No. 2, p. 1209 to 1210).
These superconducting compound oxides are expected to be utilized in actual uses of superconductors because the superconducting phenomenon can be realized with a relatively cheap cryogen of liquid nitrogen. In the early stages of development, these superconducting compound oxides were obtained as sintered bodies by solid reaction but it later became possible to prepare thin films of these superconducting compound oxide materials.
When thin films of oxide superconductor are used in electronics, it is required to prepare thin films whose crystals are well-oriented. In fact, oxide superconductors possess high anisotropy in their superconducting properties and hence it is necessary to control the crystal orientation during the film-forming stage so as to adjust to particular applications. In the thin films of oxide superconductors, higher current flows along a direction which is perpendicular to the c-axis. For example, when a thin film of oxide superconductor is used as superconducting wiring lines in a circuit, it is required to pass superconducting current along a direction which is parallel with a surface of the substrate. In this case, the c-axis of crystals of the thin film must be oriented perpendicularly with respect to the surface of the substrate. In other words, (110) oriented thin films of oxide superconductor are required in these applications. In another application such as layered superconductor-insulator-superconductor (SIS) type devices, it is required to pass superconducting current along a direction which is perpendicular to the surface of the substrate. In this case, the c-axis of the crystals of the thin film prepared must be oriented parallel with the surface of the substrate. In other words, (100) oriented thin films or (110) oriented thin films of oxide superconductor are required in these applications.
The importance of selection of the deposition plane of the substrate or optimization of deposition conditions, particularly substrate temperature, for realizing a desired crystal orientation in a thin film of oxide superconductor, was reported in many papers, including Enomoto et al., in Japanese Journal of Applied Physics, Vol. 26, No. 7, July 1987, pp. L1248-L1250, and Asano et al. in Japanese Journal of Applied Physics, Vol. 28, No. 6, June 1989, pp. L981-L983.
A predetermined crystal orientation of oxide superconductors can be realized only under predetermined conditions and on predetermined substrate materials. In fact, the substrate on which a thin film of oxide superconductor is deposited must be selected from those that have well-matched lattice constants and do not diffuse or migrate into the superconductor, such as MgO (100) single crystal or SrTiO.sub.3 (100) or (110) single crystal.
Use of these substrates in industrial applications, however, is limited because they are costly materials and are not mass produced. Further, since the diameters of these oxide single crystal substrates are limited, it is impossible to produce a thin film of oxide superconductor having a large area which will be required in the near future.
Therefore, it is proposed to use, as a substrate, a silicon single crystal (silicon wafer), which is less expensive and is available in the market, so as to prepare a thin film of oxide superconductor thereon.
However, when a thin film of oxide superconductor is deposited on a silicon wafer, the superconducting properties of the resulting thin film are seriously deteriorated or lost due to chemical reaction between the oxide of which the superconducting thin film is made and the silicon of which the substrate is made.
In order to solve this problem, it has been proposed to interpose a buffer layer between the silicon wafer and the thin film of oxide superconductor so as to prevent the reaction. However, the known buffer layers are not satisfactory as to control or adjustment of the crystal orientation of a thin film of oxide superconductor which is deposited thereon.
The present applicant proposed to interpose two intermediate layers between a silicon wafer and a thin film of oxide superconductor in U.S. patent application Ser. No. 07/806,329 which corresponds to EP patent application No. 91 403 398.0.
Therefore, an object of the present invention is to solve the problem and to provide a pre-treatment process of the silicon wafer, which permits preparation of a thin film of oxide superconductor having a desired crystal orientation on a silicon wafer.